1. Field of the Invention
The present invention relates to a method for preparing garnet single crystals represented by the composition formula CaxNbyGazO12 (2.9<x<3.1, 1.6<y<1.8, 3.1<z<3.3) by the Czochralski technique, and more particularly to a method for preparing a large-diameter single crystal without any inclusions occurring in the crystal. The single crystals prepared according to the present invention may be preferably used as a single crystal substrate on which a single crystal of bismuth-substituted rare-earth iron garnet is grown by the liquid-phase epitaxial growth.
2. Disclosure of the Related Art
In recent years, fiber-optic communication systems have been increasingly employed in trunk communication lines as well as subscriber lines because the fiber-optic communication system can transmit a larger amount of data at high speeds with lower losses as compared with conventional telecommunication systems. The fiber-optic communication system employs, as its light source, semiconductor laser devices, which are known to be susceptible to extraneous light and thereby become unstable. The semiconductor laser is now suffering more seriously from reflected light not only from the near-end but also from the far-end as the fiber-optic communication system offers communications with lower losses. To avoid the effects caused by the reflected light, the fiber-optic communication system employs optical isolators. In general, the optical isolator, which includes a polarizer, a Faraday rotator, and an analyzer, functions to transmit light in the forward direction with low losses, whereas preventing the transmission of light incident in the reverse direction.
Materials of the Faraday rotator for use with the optical isolator, an optical circulator, a magneto-optic sensor or the like include single crystal film of bismuth-substituted rare-earth iron garnet which is grown on a single crystal substrate mainly by liquid-phase epitaxial method. To grow high quality single crystal film of the bismuth-substituted rare-earth iron garnet by epitaxial growth, it is necessary to provide as small a difference in lattice constant between the single crystal film of the bismuth-substituted rare-earth iron garnet and the substrate single crystal as possible within the range from the growth temperature to the room temperature. That is, it is a necessary condition that the lattice constants and the coefficients of linear expansion of the film and the substrate should be sufficiently close enough to each other.
As a single crystal substrate that satisfies the condition, suggested in Japanese Patent Laid-Open Publication No. Hei 10-139596 is a substrate of single crystal CaxNbyGazO12 (2.9<x<3.1, 1.6<y<1.8, 3.1<z<3.3) (hereinafter may be referred to as CNGG single crystal). The CNGG single crystal described in this publication has inclusions in the crystal though no cracks are found therein. The single crystal film of the bismuth-substituted rare-earth iron garnet grown on a substrate made from the CNGG single crystal. However, although no cracks are found, microdefects are found in the grown crystal film. On the other hand, the substrate can be made avoiding the sites of inclusions in the CNGG single crystal. In this case, a large-diameter single crystal substrate cannot be provided, so that no large-diameter single crystal film of bismuth-substituted rare-earth iron garnet is obtained. As described above, it cannot be said that the CNGG single crystal substrate described in the aforementioned publication has a sufficiently high quality as the substrate for growing thereon a high-quality film of bismuth-substituted rare-earth iron garnet.
On the other hand, a substrate for forming magnetic garnet single crystal film thereon is disclosed in WO03/000963.